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Unhexunium
Unhextrium, Uht, is the temporary name for element 163. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Uht may form during neutron star mergers. NUCLEAR PROPERTIES At least one model exists predicting the half-lives and decay modes for nuclides up to Z = 175 and N = 333(1), which includes isotopes Uht 496 and lighter. It is helpful to view p 18 of Ref. 1, which maps predicted half-lives of nuclides in this region, and p 15, which maps principal decay modes. These maps are the result of large extrapolations from what can be tested, though. Half-lives are accurate only to within three orders of magnitude and decay modes give no information about competing minor decay modes. Results given in this article should be regarded as tentative. Ref. 1 predicts a band of nuclides ranging from Uht 438 to Uht 471. As neutron count increases in this band, half-life increases, reaching a maximum exceeding a second. Principal decay mode shifts from fission, to alpha emission, then to beta emission. This pattern is expected, given the predicted neutron shell closure at N = 308. However, there is also a band of alpha-decaying isotopes with millisecond-scale half-lives predicted to lie between Uht 472 and Uht 483, a zone which should be strongly destabilized by the N = 308 closure. A neutron shell closure has also been predicted to occur at N = 318(2) and a proton shell closure has been predicted at Z = 154(3). The long lives reported for Uht 472 to Uht 483 may indicate effects of those two closures. Between Uht 484 and Uhr 496, the predicted short-lived, fission-decaying isotopes and nuclear drops too short-lived to be nuclei are expected. Beyond Uhr 496, predicting nuclear properties is largely guesswork. At least two sets of predictions exist for location of the neutron dripline up to Z = 175(3),(4). These two indicate that the dripline occurs between Uht 559 and Uht 589. Neutron shell closures are also predicted to occur at N = 370(3) and 406(5). Ref. 5 also includes predicted structural correction energies provided by shell closures at N = 406 and Z = 164. Applying the Z = 164 correction at full strength predicts that Uht 459 through Uht 496 have fission half-lives exceeding 0.001 sec, which is inconsistent with the much higher-quality results of Ref. 1. Multiplying the Z = 164 corrective energy of Ref. 5 by 0.7 makes the two sets of data consistent with each other. With this adjustment, beta-decay can be expected to predominate in isotopes between Uht 498 and Uht 589, with a significant beta-decay branch occurring as low as Uht 490. The band from Uht 498 to Uht 559 is particularly important, since it lies below the lower neutron dripline prediction, strongly implying that these isotopes will beta decay. Thehe N = 370 closure should produce a band of beta-decaying isotopes in its own right, furnishing additional stability in beta-decaying isotopes between Uht 525 to Uht 535. FORMATION Polar jets emanating from young neutron stars and black holes function like giant, sloppy particle accelerators. It is possible for fusion or multinucleon transfer reactions to produce all isotopes of Uht, but only in atoms-per star quantities. This section addresses possible isotopes which can form in quantity. Material originally found 800-1000 m beneath the surface is expected to be ejected from a neutron star when it disintegrates during a merger. This material will consist of nuclides at or near the neutron dripline and having a proton count which may go as high as Z = 170. A zone of beta-decaying nuclides extending from the dripline to Uhu extends as low as A = 547, implying that the isotopes between Uht 540 and Uht 559 are likely to form in quantity (taking b+x*n decay into account), and that isotopes between Uht 560 and Uht 589 are possible, depending on where the dripline actually occurs. These nuclei will have millisecond-scale half-lives. Neutron capture is unlikely to produce more than an atom or two per star of nuclides heavier than A = 350. ATOMIC PROPERTIES Uht is predicted to be a transition metal (d block), although it is never occurs in environments cool enough to have chemistry. Of more importance, its last (1s) ionization energy appears to be in the range 695 - 805 keV, meaning that bare nuclei are abundant only at temperatures above 5 gK. Electron configuration predicted for Uht takes account of finite nuclear size, nuclear shape is neither considered nor dismissed explicitly. Nuclear shape may have some effect on electron structure. REFERENCES 1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011. 2. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics.42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105. 3. “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013. 4. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url: https://arxiv.org/abs/nucl-th/0306061 5. "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v.68, no. 7, pp 1133-1137; 2005. (02-14-20)